Select edges by length?
Yes.

Select an edge and press &#8679; ShiftG> Length. By default this will select all other edges with equal length to the active edge.
By adjusting the options in the redo panel (F6) or at the bottom of tool shelf (T), you can also select edges with greater or lesser lengths and compare lengths with a threshold to select edges that are not exactly the same length, but similar.



Once you have a partial selection on the billboards you want to select, you can select all geometry connected to the selection with &#9096; CtrlL. Or in the case of a plane, by switching to face select mode (&#9096; Ctrl&#8633; Tab) while holding &#9096; Ctrl.